Several strategies have been utilized to improve the efficacy of vaccines targeting cancers and infectious diseases (Barouch and Deeks (2014) Science 345:169-174; Coffman et al., supra; and Koff et al. (2013) Science 340(6136):1232910). Blockade of inhibitory pathways to boost immune responses, such as via the inhibition of the co-stimulatory molecules PD-1 and PD-L1, has been studied for the treatment of human cancer and chronic infectious diseases (Ha et al. (2008) J Exp Med 205:543-555; and Pardoll et al. (2012) Nat Rev Cancer 12:252-264. For example, several studies in murine models, non-human primates, and human clinical trials have shown that preventing interaction of PD-1 receptor and its ligand, PD-L1 and PD-L2, with a blocking antibody (either alone or in combination with other blocking antibodies toward other T cell co-inhibitory molecules) resulted in improved T cell function and reduction in viral loads (Brahmer et al. (2010) J Clin Oncol 28:3167-3175; Porichis et al. (2011) Blood 118:965-974; and Zhang et al. (2007) Blood 109:4671-4678). Therefore, manipulation of costimulotory pathways offers great potential to modulate immune responses in humans.